Page images
PDF
EPUB

In building edifices which are to support great weights, whether a church, a house, or a bridge, the primary and indispensable consideration of the architect is to obtain a permanently firm and stable foundation ; well knowing that unless this be first substantially made, no future dependence can be placed on the stability of the intended superstructure : but this most requisite precaution has but recently been attended to in the formation of roads, and only on those roads in Scotland, and between London and Holyhead, which have been under the direction of Mr. Telford.

If the foundation of a road be not sufficient and equal to the pressure it has to sustain, the whole fabric, though in other respects ever so well constructed, must fail in permanent stability, and the hardness of it will be imperfect from its elasticity.

Having now stated all that the rules of science relating to moving bodies suggest, in order to defend the principles of road making, which have been laid down as the proper principles to be adopted, we shall now proceed further to illustrate and support these principles, by referring to experiments of the force of traction on different kinds of roads. These experiments have

[ocr errors][merged small]

been made with the machine invented by Mr. Macneill, which has been already mentioned, and may be relied upon for their accuracy, in consequence of their having been carefully examined by several very eminent civil engineers.

These experiments uniformly show, that the force of traction is, in every case, nearly in an exact proportion to the strength and hardness of a road. The following are the results : on a wellmade pavement, the power required to draw awaggon is 33 lbs.; on a road made with six inches of broken stone of great hardness, laid on a foundation of large stones, set in the form of a pavement, the power required is 46 lbs. ; on a road made with a thick coating of broken stone, laid on earth, the power required is 65 lbs.; and on a road made with a thick coating of gravel, laid on earth, the power required is 147 lbs.* Thus it appears that the results of actual experiments fully correspond with those deduced from the laws of science.

It has been considered necessary to enter into these details in showing that no road can be correctly called a good road unless it is so constructed as to be a very strong and a very hard one, be

[ocr errors]

* See Appendix, No. I.

ness.

cause all the main roads of the kingdom are still very defective in respect to strength and hard

This is a fact which cannot be disputed; first, because there is always mixed up with the

, body of materials, which forms the crust of every road, a great quantity of earth ; secondly, because this crust is every where too thin ; and, thirdly, because it very frequently lies upon an elastic substratum. Although there may

be exceptions, this may be taken as an accurate description of the general state of the roads.

Notwithstanding all the roads are now much better than at any former period, and may deserve to be called good roads, in comparison with the roads of ten or fifteen years ago; when it is considered how much better they would be if they were reconstructed with a proper foundation coated with broken stones of great hardness, they should still be set down as being imperfect roads. Let any road trustee or surveyor who doubts this, reconstruct a mile of a road, now considered an excellent one, with a bottoming of pavement, coated with hard stones, and no stage coachman who shall drive over it will hesitate to bear testimony to the increased ease with which his horses do their work upon it. The explanation which has been given in this chapter of the laws of motion, as applicable to the subject of road making, and particularly of the effect of an elastic substratum of a road, as stated by Professor Leslie, in consuming the moving force, and adding to the horse's labour, is quite conclusive in showing how much at variance to the first principles of science the following doctrines are, which are to be found in some modern publications.

“ That a foundation or bottoming of large stones is unnecessary and injurious on any kind of subsoil.”

“ That the maximum strength or depth of metal requisite for any road, is only ten inches.”

“ That the duration only, and not the condition of a road, depends upon the quality and nature of the material used.”

“ That free stone will make as good a road as any other kind of stone."

“ That it is no matter whether the substratum be soft or hard.” *

* The passages marked with inverted commas have been extracted from the publications of Mr. M‘Adam.

As many persons advocate Mr. M'Adam's doctrine of elastic roads, it may serve to show the real value of it,

1 1

Mr. Wingrove, an eminent practical road surveyor, observes, in a Treatise on the Bath roads, after quoting these sentences, “that with

by putting it in juxta-position with that of the celebrated natural philosopher, the late Professor Leslie. Extract from the Evidence of Mr. Extract from Pro

M Adam. (Remarks on Road-Making, fessor Leslie's p. 111.)

Elements of Natural Philo.

sophy." “What depth of solid materials would “The resistance you think it right to put upon a road in which friction ocorder to repair it properly ?-I should casions (to carrithink that ten inches of well consolidated ages) partakes of materials is equal to carry any thing. the nature of the

“ That is, provided the substratum is resistance of fluids: sound?- No: I should not care whether it consists of the the substratum was soft or hard: I consumption of the should rather prefer a soft one to a hard moving force, or

of the horse's la“ You don't mean to say you would bour, occasioned prefer a bog?— If it was not such a bog by the soft surface as would not allow a man to walk over, I of the road, and should prefer it.

the continually de“ But must not the draught of a car- pressing of the riage be much greater on a road which spongy and elastic has a very soft foundation than on one substrata of the which is of a rocky foundation ?- I road." think the difference would be very little indeed, because the yield of a good road on a soft foundation is not perceptible.”

one.

« PreviousContinue »